1. Field of the Invention
The present invention concerns providing an integrity alert to a user of the Global Positioning System (GPS) when the signal contains an error. Specifically, the invention concerns alerting the user of the signal error by shifting the phase of the Global Positioning System (GPS) signal from an initial value by a predetermined phase-shift amount.
2. Description of the Related Art
The use of the Global Positioning System (GPS) is increasing rapidly in a variety of applications. Accordingly, the users of GPS expect to have, and often must have, confidence in the integrity and accuracy of the GPS signal. U.S. Pat. No. 6,603,426, (hereinafter “the '426 patent”), issued Aug. 5, 2003, sets forth one solution to this problem by the same applicant of the present invention.
As explained in the Background section of the '426 patent, the Global Positioning System (GPS) consists of 24 earth-orbiting satellites. The GPS satellites broadcast a navigation message via a radio frequency (RF) signal. This signal allows any individual with a GPS receiver to process the GPS signals and determine his or her precise longitude, latitude, altitude, velocity and time anywhere in the world.
Although the Global Positioning System (GPS) provides very accurate position and time information, there are times when GPS satellite system malfunctions can introduce errors into the GPS signal transmitted from the GPS satellite. When this occurs, the GPS receivers will not be able to accurately determine position and/or time. Past data has shown that the GPS signal has typically malfunctioned on the average of around 45 minutes a year. When the GPS satellite system is functioning properly and producing accurate GPS data, the GPS data is described as having “integrity.”
GPS signal errors can be caused by a number of conditions. For example, if one of the GPS satellite transmitter elements, or any of the other satellite components, fail, the GPS signal waveform can become corrupted. For example, an output amplifier in the GPS satellite may start to malfunction and thereby corrupt the transmitted signal. Another source of error is a failure of the satellite's atomic clock. If a clock failure occurs, the satellite will transmit incorrect time data and introduce error into the computed position information. Another potential error is the transmission of erroneous correction data from GPS ground stations to GPS satellites. GPS ground stations uplink correction data to the GPS satellites every 24 hours. If a ground station sends the wrong correction data, then the GPS satellites will produce inaccurate or erroneous output signals. As an example of this type of error, a ground station could mistakenly send correction data for Tuesday when it was supposed to send correction data for Wednesday.
Errors in GPS signals can lead to severe safety issues or inefficient operation for many systems that use GPS signals, such as aircraft systems, transportation systems, weapon systems and so forth. New aircraft navigation systems are being developed which rely on GPS signals for navigation. It can be appreciated that errors in the received GPS signal in an aircraft navigation system could lead to mid-air crashes or other dangerous situations. The Federal Aviation Administration (FAA) has a goal of having no more than a 2 times 10−8 probability of error in the GPS signal without an alert that the signal is hazardous or misleading. With the current GPS system, the probability of error in the GPS signal is on the order of 10−4 per satellite per hour or even higher. Thus, for example, it is possible that a current GPS satellite could produce a probability of error ten thousand times higher than the FAA's desired goal.
One of the proposed systems that will utilize GPS is an intelligent highway system. Intelligent highway systems would use GPS signals to manage traffic by providing autonavigation for the automobiles on the freeways. Similar systems have been proposed for trains. Thus, it will be a very important safety issue for these systems to ensure the integrity of the received GPS signals.
Currently, the Global Positioning System (GPS) system does not have any form of integrity monitoring as part of the system. A system known as the “Wide Area Augmentation System” (WAAS) is currently being designed and developed to provide integrity monitoring of GPS. The WAAS will use a series of new ground stations at known locations all over the world. Each ground station will include a satellite antenna which receives GPS signals from the in-view GPS satellites. Each ground station will use these GPS signals to calculate its own position. By comparing the calculated position with the known position of the ground system, the accuracy and the integrity of the GPS signal of each corresponding in-view GPS satellite can be determined.
If the calculated position is different from the known position of the WAAS ground station, a correction message is generated by the ground station. The ground station transmits the correction message to an independent messaging system, such as a geosynchronous satellite. This geosynchronous messaging satellite then broadcasts the correction message to all GPS users in the region. The GPS users then use the correction message to correct the GPS data received from the GPS satellites. Alternatively, the geostationary messaging satellite can transmit an integrity message to all GPS users in the region, informing the users of a potential satellite malfunction. GPS users can thereby be informed that they should not rely on the GPS signals being received. Alternatively, the ground system could send the integrity message to a mission control system which sends a message to the GPS satellites to correct the erroneous data or to cease transmitting all GPS navigation data.
The WAAS integrity monitoring system will require an enormous cost including the cost of building the new WAAS ground stations, procuring the new geostationary messaging satellites, and the costs of maintaining and operating the ground stations. Estimated costs for the development and implementation of WAAS are greater than 2 billion dollars. Moreover, the WAAS may not be able to signal a problem with GPS integrity with sufficient speed. Many systems, such as aircraft navigation systems, using GPS frequently need to know of a change in GPS signal integrity in times less than 1 sec after a malfunction or error occurs.
The '426 patent offers one solution to the GPS integrity problem by having each GPS satellite verify the accuracy and/or integrity of its own operations by calculations and processing internal to the GPS satellite and by crosslink communications with other GPS satellites in the GPS constellation, after which the GPS satellite transmits an integrity message to all GPS users in view of the satellite, the integrity message being either incorporated into the existing GPS navigation message, or being transmitted over a separate communication channel. This solution offers the GPS user the ability to monitor the GPS integrity message to determine whether any errors exist in the received GPS navigation signal. Although the solution of the '426 patent provides a reliable way for GPS users to be alerted to inaccuracies in the GPS signal, the solution requires the GPS user to read the GPS integrity message to determine if any error exists, and the magnitude of the error. Time is required to read the integrity message, and if the integrity message is transmitted over a separate communication channel, the GPS user must have an upgraded receiver capable of receiving the separate communication channel.
What is needed is a system that can provide a high-level of confidence in GPS integrity by alerting GPS users to a loss of GPS signal integrity with sufficient speed to satisfy safety concerns and regulatory standards, without the need for using a separate communication channel, and without the enormous cost and complexity associated with a system such as the WAAS.